In 333 B.C., Alexander the Great marched his army into the city of Gordion, where there was a massive knot in desperate need of untying. Legend held that the hero who could undo this exceedingly intricate Gordian Knot, as it was not-so-creatively called, would rule Asia. Alexander, unable to unravel the knot, drew his sword and sliced right through it, then, apparently with the blessing of the eviscerated loops, went on to conquer Asia minor.
The tall tale gives us the expression “to cut the Gordian Knot,” meaning to solve a seemingly insurmountable problem by over-the-top means. It also lends its name to one of the animal kingdom’s most clever parasites, the Gordian worm, which has solved the often insurmountable problem of survival with means that are horrifyingly over-the-top.
More commonly known as the horsehair worms, because folks with a limited understanding of reality once thought they were horsehairs that animated upon hitting water, the 350 or so known species invade insects like the luckless cricket above. After developing for several months, the worms mind-control their hosts to make a kamikaze dive into water, then escape through holes bored in the insect's exoskeleton. The parasites end up in a tangled knot that can be as heavy as the tattered—and oftentimes very much alive—host they leave behind.
All across America in rivers or streams, horsehair worm eggs hatch and settle lazily to the bottom as larvae (we'll be talking specifically about the species Paragordius varius and its parasitism of crickets). Unable to swim up the water column, the larva simply wait to be eaten by the larvae of other insects like midges, mayflies, and mosquitoes. When these insects metamorphose and emerge from the water, they live out their aerial lives with the larva in tow, then inevitably croak and get snatched up by a cricket, according to parasitologist Ben Hanelt of the University of New Mexico.
Once the worm larvae find themselves in the insect, “they will penetrate through the gut of the cricket and get into the body cavity, where they then grow from a tiny, tiny larva to something that’s now on the average of a foot long,” he said. (There’s a 6-foot species, by the way, that parasitizes an as-yet-unknown insect, probably a giant and perpetually nervous cockroach.)
Really, the horsehair worm is nothing more than a giant gonad wrapped in a thin sheath of muscles, and I say that with all due reverence. Curiously, they don’t even have a mouth to eat with or chew their way through the cricket, so Hanelt remains unsure how they bore into the body cavity, and then through the exoskeleton to escape.
And there’s no digestive system as we would recognize it, because like the tapeworms that take up residence in our guts, they’re living in a veritable sea of food. “The way that these guys actually get their nutrients is right through the cuticle,” said Hanelt. “Right through the skin of the worm is where the fat and the sugar is actually absorbed straight from the body fluids of the host.”
Now, it’s nearly impossible to identify an infected cricket, for this is no clumsy zombie of popular culture. Outwardly, the cricket behaves quite normally, save for a brilliant little trick the worm plays: It manipulates them to shut the hell up with the chirping. Chirping is, after all, energetically expensive, not to mention a real fine way to get yourself noticed and eaten, a rather anticlimactic end to the worm’s grand scheme.
When the worm is ready to leave the cricket, though, you’ll know it. Typically crickets give running water a wide berth, instead getting their hydration from food and the occasional dew drop. According to Hanelt, you can take a non-infected specimen and drop it near running water and it’ll leg it right out of there, every time. The dangers of hungry fish and drowning are simply too great.
But a cricket infected with a horsehair worm swears, quite wrongly, that it's a great swimmer. At the behest of the worm it seeks out bodies of water with its antennae, which pick up the slightest changes in humidity. Then, seemingly against its better judgment, the host proceeds to perform a sicknasty cannonball: “If you take a cricket that actually has a worm in it,” said Hanelt, “and put it next to the water, it will always, in every case, jump immediately in.”
After admiring the cannonball, the worm, monitoring the world through a porthole it bored in the cricket, makes its move, squirming out of its host as soon as it hits the water. In nature, it’s typically one worm per cricket, though every now and then two or three will emerge. In Hanelt’s lab, however, his record is an astonishing 32 worms erupting from one unfortunate host (that GIF at top, which I ain’t even about to apologize for, was half that many worms).
The parasite, now free, will swim around in search of a mate. When they pair up, the male aligns his cloaca with the female and passes his sperm. Having served his sole earthly purpose, he will die. The female goes on to lay as many as 15 million eggs, which she pastes underwater on a stick or stone. When she’s done, she too will die, emptied of eggs and totally flattened out like a straw wrapper that’s lost its straw. Two weeks later, her eggs hatch into the larvae that settle once more onto the river bottom, beginning the process anew.
It’s a remarkable tale of an organism adapting over evolutionary time to manipulate another and use it as a private escort. But how on Earth can the worm hijack a cricket’s brain? And why would this evolve in the first place?
“First of all,” said Hanelt, “the worm appears to be producing large amounts of neurotransmitters,” chemicals that allow the transmission of signals between neurons. “And the neurotransmitters that it's producing are thought to make the cricket basically act in ways that normally the cricket wouldn't act. And exactly which neurotransmitters these are and how they're affecting the crickets, that we don’t know.” Secondly, it appears the worm triggers the cricket to boost production of neurotransmitters. But there is still much, much to be learned, just as there is with other highly sophisticated mind-controllers like wasps that enslave cockroaches and fungi that zombify ants.
As for why the worms would have needed to evolve such tactics, we have only theories. For Hanelt's money, it's a matter of opportunism. In the high deserts of New Mexico, he finds horsehair worms aplenty. Wandering through an extremely dry forest, he'll come across a dinner-plate-sized puddle, and sure enough, there they squirm. "When I look around, I see very few resources," he said, "flowering plants, grasses, etc. So, if I was a worm, the best way to make a living out here is to get into a very nutritious insect host, which is filled with fat. This represents the easy life."
The cricket certainly gets a raw deal, but all is not necessarily lost for the host. Once free of its parasites, it can drown or fall prey to a fish—or, incredibly, escape a watery grave and live out the rest of its life as if it didn't just give grueling birth to its own weight in worms. Indeed, Hanelt hears from folks all the time who find worms in toilets or dog bowls with no sign of their host, which has likely shuffled away exhausted, muttering to itself, head hung low. And that unfortunate cricket in the GIF that I still don't regret? She not only survived, she went on to produce viable eggs.
“I always tell students this way to think about it,” said Hanelt. “Imagine if I told you to walk over to your car and remove half of its weight, but still have the car be able to get you to the airport. And somehow these worms have figured out how to do that within the cricket host, that they're able to take half of everything that’s within that cricket but still make it tick. It's kind of amazing.”
And so it would seem—with Gordian worms, at least—that the massive release of neurotransmitters is mightier than the sword.
Special thanks to ecologist Larry Serpa of the Nature Conservancy for suggesting this week's creature. Browse the full Absurd Creature of the Week archive here. Have an animal you want me to write about? Email matthew_simon@wired.com or ping me on Twitter at @mrMattSimon.
